Newer versions of the retryablehttp package use a context, so we need to
add that in our custom `CheckRetry` function.
In addition I removed the `return true, nil` to continue retrying in
case of an error, and instead directly call the `DefaultRetryPolicy`.
This is because the `DefaultRetryPolicy` will now also take the context
into consideration.
The state manager refactoring in an earlier commit was reflected in the
implementations of these backends, but not in their tests. This gets us
back to a state where the backend tests will compile, and gets _most_ of
them passing again, with a few exceptions that will be addressed in a
subsequent commit.
The new config loader requires some steps to happen in a different
order, particularly in regard to knowing the schema in order to
decode the configuration.
Here we lean directly on the configschema package, rather than
on helper/schema.Backend as before, because it's generally
sufficient for our needs here and this prepares us for the
helper/schema package later moving out into its own repository
to seed a "plugin SDK".
